JDR13, Quarl's up to pack 3 now, is the best if not its definatly second, well last time I looked.
One thing you might consider is getting BKMod it changes the crappy console looks and functions of the Inventory, Map, Quest, Weapons and Spell Tabs making them much more tolorable, it may very well be the number most popular mod.
Here is a link to a great site which has many top modders whom have fled the Bethseda borads due to crappy rules like no nudity, look for Oblivion "Work in Progress' and "Release".
Quarl, Eshme and Aleenee a couple I recall, atm.http://canadianice.ufrealms.net/forum2/index.php

Over the last week I spent about 2 hours reaquainting myself considering play through again while I wait on Gothic 3 and these are some of the things I noticed.

I personally couldn't play without all the Better Body, Face and Hair Mods then make the PC look so damn good.
The site http://canadianice.ufrealms.net I gave you has all the greats (well many) doing all the great new armors and weapons, Better Bodies has a Whole damn store to shop at for Wickedly good outfits and you can choose the Non nudity version if you don't want to catch a peep while switching outfits.

Last my favorite is Gabby companion Mod, some of the Compainons are amazing with back stories, quest, riding horses, collecting plants, organising them for you to make potions, they will wait for you doing things like making potions or wait for you in the local tavens, going to get food or shopping during the day until you return.
Though you will need to spend some time with them, which could be a little overwhelming if you haven't play it before.
Its great having a companion though Gabby even tells you what her Stats are if you ask, so you can monitor how she is leveling choosing how to focus her.
When Stealthing she cast Invis on herself, this may seem a little unfair but it was the best way to prevent a companion form gettting argo if your a Stealth style character.
She is essential so she will only get knocked out in battle but you can turn it off, by now they may have a resurction spell depending on how strict of a player you are.

Well anyway this should be at least a start for you.

With a great editor Gothic 3 could blow Oblivion out of the water, these mods are what make Oblivon.